Blast Protection Damper

ABSTRACT

A blast wave damper comprises a section of duct ( 14 ), with a multiplicity of rigid elements ( 18 ) each extending across the duct. All the elements are parallel to each other, and they are arranged in an array consisting of a multiplicity of lines, each such line extending across the duct ( 14 ). The elements in one line are staggered relative to the elements in an adjacent line, and the gaps between successive elements within a line are no wider than the widths of the elements. There may be ten columns of tubular elements in a regular array. This may be combined with a louvre mechanism ( 20 ) arranged to shut if the pressure drop exceeds a threshold.

The present invention relates to a damper for mitigating blast waves in a duct.

It is known that ignition of a flammable mixture in a duct can create a blast wave which propagates along the duct. This is a particular issue in oil or gas production platforms, where such flammable mixtures may arise. If such a blast wave propagates into a region where there are people, this can cause significant injury, such as burst eardrums or damaged lungs. The provision of louvres to inhibit such blast waves is known, but louvres cannot shut sufficiently quickly to prevent passage of the pressure wave.

According to the present invention there is provided a blast protection damper comprising a section of duct, with a multiplicity of substantially rigid elements each extending across the duct, the elements all extending parallel to each other and being arranged in an array consisting of a multiplicity of lines of said elements, each such line extending across the duct, the elements in one line being staggered relative to the elements in an adjacent line, and the gaps between successive elements within a line being no wider than the widths of the elements.

The damper is particularly suitable for ducts through which, in normal operation, there is a forced gas flow. For example this may be a flow of air for cooling or for ventilation, and typically the flow velocity in normal operation is in the range between 1 and 5 m/s. The damper of the invention is to be distinguished from sound attenuators, as the elements in the blast damper are rigid and are not of a sound-absorbing or attenuating material. Such rigid materials may be characterized as those for which the characteristic acoustic impedance (the product of sound velocity and density) is greater than 10×10⁶ kg m⁻² s⁻¹, and more preferably greater than 30×10⁶ kg m⁻² s⁻¹. They may for example be tubes of steel or titanium or a titanium alloy.

Preferably the lines are straight lines, and within each line the elements are equally spaced. For example the lines may be columns extending between the bottom and the top of the duct. Preferably there are at least eight such lines of elements in the array, more preferably at least ten such lines, but preferably no more than fifteen. It will be appreciated that the more lines of elements are provided, the greater the pressure drop during normal use of the duct, so there is a disadvantage in providing excessive numbers of lines of elements. On the other hand, the more lines of elements are provided, the more effective the damper is at mitigating blast waves. The optimum number of lines appears to be about ten.

Preferably the elements are of cylindrical shape, and the elements are preferably tubular, as this reduces weight while providing adequate strength. In a preferred embodiment the elements are tubes of diameter about 60 mm, and are arranged at centre-to-centre spacings of no more than 120 mm, for example 100 mm so that the elements of a single column occupy about 60% of the projected area.

Preferably the blast protection damper also includes a louvre which is arranged to shut if the pressure in the duct exceeds a threshold. This may for example incorporate a mechanical latch arranged to hold the louvre blades in an open position, but the louvre blades being oriented such that the flowing gases within the duct urge the blades towards the closed position. If the pressure in the duct exceeds a threshold indicative of the presence of a blast wave, then the mechanical latch releases the blades, which move into the closed position under the combined effect of gravity and the gas pressure.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1 shows a side elevation of a blast protection damper of the invention; and

FIG. 2 shows an end of view of the damper of

FIG. 1 in the direction of arrow 2.

Referring to FIG. 1, a blast protection damper 10 forms part of a square duct 12 of width and height 1.2 m, through which the normal gas flow direction (and the potential blast wave direction) are indicated by the arrow A. The damper 10 consists of a section of square steel duct 14 which is of width and height 1.2 m, and of length 1.2 m, made of 5 mm-thick steel sheet, and connected to the duct 12 by means of flanges 16 at each end. There are 115 steel tubes 18 each of external diameter 60.3 mm and of wall thickness 3 mm which extend horizontally between opposite sides of the duct 14, being welded into corresponding holes in the sheets forming the duct wall, and being supported by corresponding holes in a vertical support plate 19 (shown in broken lines in FIG. 2) halfway across the duct 14. The tubes 18 are not indicated in FIG. 2, for clarity.

The tubes 18 are arranged in vertical columns, within each column the tubes 18 being spaced apart centre-to-centre (y) at 100 mm, and the longitudinal distance (x) between the centres of successive columns also being 100 mm. There are twelve tubes 18 in the first column, so that the gaps between adjacent tubes are about 40 mm, and similarly there are gaps of about 20 mm between the top and bottom tubes 18 and the top and bottom walls of the duct 14. The arrangement of the tubes in the other odd-numbered columns is identical to that in the first column. The even-numbered columns each have eleven tubes 18, and the tubes 18 are staggered relative to those in the odd-numbered columns, so that the centres of the tubes 18 are exactly midway in height between those of the tubes in the odd-numbered columns.

At the downstream end of the array of tubes 18 is a louvre mechanism 20, consisting of eight louvre blades 22 (shown in broken lines in FIG. 1) which in normal operation (as shown) are oriented at about 45°, each of length about 190 mm, whose upper edge is fixed to a 25 mm diameter steel rod 24. The steel rods 24 extend through bearings in the side walls of the duct 14, and are welded to arms 25 all of which are pivotally connected to a link bar 26. The link bar 26 is secured by a spring-operated latch mechanism (not shown) in a box 27, such that if the pressure forces acting on the blades 22 exceeds a threshold value, then the latch is released, so that the blades 22 rotate into a generally vertical position, under the effect of both the gas pressure and gravity. In the resulting, closed, position of the louvre mechanism 20 the bottom edge of each blade 22 rests against the steel rod 24 of the next blade down, apart from the lowest blade 22 which rests against a stop 28.

In normal operation the damper 10 imposes a comparatively small pressure drop on the normal gas flow along the duct 12. In the event of a blast wave, the wave is scattered by the tubes 18 and dissipated, to a large extent. For example in the event of a blast wave with a pressure increase up to one atmosphere (100 kPa), for a blast duration of 200 msec, the damper 10 restricts the over pressure to about 15.3 kPa at a location 2 m downstream of the damper 10. The pressure increase can be further limited by providing a plenum of a larger cross-sectional area than the duct 12 downstream of the damper 10.

It will be appreciated that the embodiment described above is given by way of example only. There may be some situations in which the louvre mechanism 20 can be omitted, depending on what magnitude of blast waves is expected.

It will also be appreciated that a damper of the invention can be sized to suit a particular duct. For example, for use with a smaller duct there may be a similar damper with the same number of columns of tubes, but all the dimensions being correspondingly smaller. Alternatively the tubes might be of the same size and spacing as described above, but the numbers of tubes in each column being reduced in accordance with the size of the duct. In either case the preferred number of columns is between eight and twelve, more preferably ten, if the damper is to be suitable for blast waves with a pressure increase of 100 kPa. If the damper is for use in situations in which the blast wave pressure will not exceed say 50 kPa, then the number of columns could be reduced; while if the damper is to contend with blast waves pressure up to say 150 kPa, then the number of columns would preferably be increased. 

1. A blast protection damper comprising a section of duct, with a multiplicity of substantially rigid elements each extending across the duct the elements all extending parallel to each other and being arranged in an array consisting of a multiplicity of lines of said elements, each such line extending across the duct, the elements in one line being staggered relative to the elements in an adjacent line, and the gaps between successive elements within a line being no wider than the widths of the elements.
 2. A damper as claimed in claim 1 wherein the lines are straight lines, and within each line the elements are equally spaced.
 3. A damper as claimed in claim 1 wherein there are between eight and fifteen lines of elements in the array.
 4. A damper as claimed in claim 3 wherein there are ten lines of elements in the array.
 5. A damper as claimed in any one of the preceding claim 1 wherein the elements are of cylindrical shape.
 6. A damper as claimed in claim 1 wherein the elements within a single line occupy at least 50% of the projected area.
 7. A damper as claimed in claim 1 also comprising a louvre which is arranged to shut if the pressure in the duct exceeds a threshold.
 8. A damper as claimed in claim 5 wherein the elements are tubular. 